Angle-Independent Surface-Instability Hydrogel Sensors Enabled by Thickness Control

Surface instability-based hydrogel sensors offer equipment-free visual detection but suffer from severe angle-dependent visibility that limits practical applications. Here we demonstrate that film thickness serves as a critical fourth design parameter—beyond elastic modulus, swelling ratio, and stimulus concentration—to achieve omnidirectional optical readout. Through systematic optimization across poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA), poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)), and poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAm-co-Am)) systems, we identify an operational thickness window of 160–1300 μm, with 300 μm producing wrinkle dimensions that match visible light wavelengths for angle-independent scattering. This morphological control reduces viewing angle effects by >80%, enabling reliable naked-eye detection across a wide viewing angle range (0–60°), representing an >80% improvement over previous angle-dependent sensors. We validate this design framework through enzyme-coupled glucose sensors operating across clinical ranges (200–1000 mg/dL) and direct polarity sensors with binary discrimination at Polarity Index (PI) ~3.5. Exploiting the threshold nature of instability-induced scattering (IIS), we further demonstrate soft material logic gates (AND, OR, XOR) that process environmental stimuli and autonomously control LCE actuators within 15 s—establishing the examples of hydrogel-based Boolean computation without electronic components. These advances transform IIS sensors from laboratory curiosities into practical platforms for soft robotics and autonomous systems.